This is a proposal to test the hypothesis that Fragile-X mental retardation results from a failure of FMRP to bind specific RNAs, and to identify those RNAs. The foundation of this proposal is our finding of sequence and structure-specific targets for the FMRP RGG box and KH2 domain, the latter of which is associated with severe disease in a patient with a missense mutation (I304N) in this RNA-binding domain. We have related the KH2 RNA target, termed a kissing complex RNA, to disease by demonstrating that it competes FMRP off of brain polyribosomes, suggesting that this RNA motif is used by FMRP to effect translational regulation of specific mRNAs. Three aspects of FMRP sequence-specific RNA binding will be definitively addressed here. First, FMRP RNA targets will be identified using several methods: bioinformatic screens complemented by structural studies, microarray analysis of coimmunoprecipitating RNAs, microarray analysis of RNAs whose distribution on polyribosomes is shifted in the absence of FMRP, and cross-linking IP (CLIP) studies. Second, to help validate these new RNA targets in vivo, we will generate three new mouse models of Fragile-X mental retardation. Third, these new model systems will be used in conjunction with the existing FMR1-null mouse to validate identified RNA targets of FMRP in mouse brain. Taken together, these studies will address the hypothesis that mental retardation associated with the I304N mutation, and likely the Fragile-X syndrome more generally, may relate to a crucial role for RNAs harboring the kissing complex motif as targets for FMRP translational regulation.